Julian day From Wikipedia, the free encyclopedia "Julian date" redirects here. For dates in the Julian calendar, see Julian calendar. For day of year, see Ordinal date. For the comic book character Julian Gregory Day, see Calendar Man. Not to be confused with Julian year (astronomy). Julian day is the continuous count of days since the beginning of the Julian Period used primarily by astronomers. The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Greenwich Mean Time, with Julian day number 0 assigned to the day starting at noon on January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar),[1] a date at which three multi-year cycles started and which preceded any historical dates.[2] For example, the Julian day number for the day starting at 12:00 UT on January 1, 2000, was 2,451,545.[3] The Julian date (JD) of any instant is the Julian day number for the preceding noon in Greenwich Mean Time plus the fraction of the day since that instant. Julian dates are expressed as a Julian day number with a decimal fraction added.[4] For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is 2,456,293.520833.[5] The Julian Period is a chronological interval of 7980 years beginning 4713 BC. It has been used by historians since its introduction in 1583 to convert between different calendars. 2015 is year 6728 of the current Julian Period. The next Julian Period begins in the year 3268 AD. Contents 1 Terminology 2 Variants 3 History 4 Calculation 4.1 Converting Julian or Gregorian calendar date to Julian Day Number 4.2 Finding Julian date given Julian day number and time of day 4.3 Finding day of week given Julian day number 4.4 Julian or Gregorian calendar from Julian day number 4.5 Julian Period from indiction, Metonic and solar cycles 5 See also 6 References Terminology The term Julian date may also refer, outside of astronomy, to the day-of-year number (more properly, the ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry,[6]— or it may refer to dates in the Julian calendar. For example, if a given "Julian date" is "October 5, 1582", this means that date in the Julian calendar (which was October 15, 1582, in the Gregorian calendar — the date it was established). Outside of an astronomical or historical context, if a given "Julian date" is "40", this most likely means the fortieth day of a given Gregorian year, namely February 9. But the potential for mistaking a "Julian date" of "40" to mean an astronomical Julian Day Number (or even to mean the year 40 AD in the Julian calendar, or even to mean a duration of 40 astronomical Julian years) is justification for preferring the terms "ordinal date" or "day-of-year" instead. In contexts where a "Julian date" means simply an ordinal date, calendars of a Gregorian year with formatting for ordinal dates are often called "Julian calendars",[6] in spite of the potential for misinterpreting this as meaning that the calendars are of years in the Julian calendar system. Historical Julian dates were recorded relative to GMT or Ephemeris Time, but the International Astronomical Union now recommends that Julian dates be specified in Terrestrial Time, and that when necessary to specify Julian dates using a different time scale, that the time scale used be indicated when required, such as JD(UT1). The fraction of the day is found by converting the number of hours, minutes, and seconds after noon into the equivalent decimal fraction. Time intervals calculated from differences of Julian Dates specified in non-uniform time scales, such as Coordinated Universal Time (UTC), may need to be corrected for changes in time scales (e.g. leap seconds).[4] Variants Because the starting point or reference epoch is so long ago, numbers in the Julian day can be quite large and cumbersome. A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision. In the following table, times are given in 24-hour notation. In the table below, Epoch refers to the point in time used to set the origin (usually zero, but (1) where explicitly indicated) of the alternative convention being discussed in that row. The date given is a Gregorian calendar date if it is October 15, 1582, or later, but a Julian calendar date if it is earlier. JD stands for Julian Date. 0h is 00:00 midnight, 12h is 12:00 noon, UT unless otherwise specified. Name Epoch Calculation Current value Notes Julian Date 12h Jan 1, 4713 BC 2457308.58472 Reduced JD 12h Nov 16, 1858 JD − 2400000 57308.58472 [7][8] Modified JD 0h Nov 17, 1858 JD − 2400000.5 57308.08472 Introduced by SAO in 1957 Truncated JD 0h May 24, 1968 floor (JD − 2440000.5) 17308 Introduced by NASA in 1979 Dublin JD 12h Dec 31, 1899 JD − 2415020 42288.58472 Introduced by the IAU in 1955 Lilian date Oct 15, 1582[9] floor (JD − 2299159.5) 158149 Count of days of the Gregorian calendar Rata Die Jan 1, 1[9] proleptic Gregorian calendar floor (JD − 1721424.5) 735884 Count of days of the Common Era Unix Time 0h Jan 1, 1970 (JD − 2440587.5) × 86400 1444701721 Count of seconds[10] Mars Sol Date 12h Dec 29, 1873 (JD − 2405522)/1.02749 50400.99828 Count of Martian days The Modified Julian Date (MJD) was introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik via an IBM 704 (36-bit machine) and using only 18 bits until August 7, 2576. MJD is the epoch of OpenVMS, using 63-bit date/time, postponing the next Y2K campaign to July 31, 31086, 02:48:05.47.[11] The MJD has a starting point of midnight on November 17, 1858 and is computed by MJD = JD - 2400000.5 [12] The Truncated Julian Day (TJD) was introduced by NASA/Goddard in 1979 as part of a parallel grouped binary time code (PB-5) "designed specifically, although not exclusively, for spacecraft applications." TJD was a 4-digit day count from MJD 40000, which was May 24, 1968, represented as a 14-bit binary number. Since this code was limited to four digits, TJD recycled to zero on MJD 50000, or October 10, 1995, "which gives a long ambiguity period of 27.4 years". (NASA codes PB-1—PB-4 used a 3-digit day-of-year count.) Only whole days are represented. Time of day is expressed by a count of seconds of a day, plus optional milliseconds, microseconds and nanoseconds in separate fields. Later PB-5J was introduced which increased the TJD field to 16 bits, allowing values up to 65535, which will occur in the year 2147. There are five digits recorded after TJD 9999.[13][14] The Dublin Julian Date (DJD) is the number of days that has elapsed since the epoch of the solar and lunar ephemerides used from 1900 through 1983, Newcomb's Tables of the Sun and Ernest W. Brown's Tables of the Motion of the Moon (1919). This epoch was noon UT on January 0, 1900, which is the same as noon UT on December 31, 1899. The DJD was defined by the International Astronomical Union at their meeting in Dublin, Ireland, in 1955.[15] The Lilian day number is a count of days of the Gregorian calendar and not defined relative to the Julian Date. It is an integer applied to a whole day; day 1 was October 15, 1582, which was the day the Gregorian calendar went into effect. The original paper defining it makes no mention of the time zone, and no mention of time-of-day.[16] It was named for Aloysius Lilius, the principal author of the Gregorian calendar.[17] Rata Die is a system used in Rexx, Go and Microsoft .NET. It uses the local timezone, and day 1 is January 1, 1, that is, the first day of the Christian or Common Era in the proleptic Gregorian calendar.[18] The Heliocentric Julian Day (HJD) is the same as the Julian day, but adjusted to the frame of reference of the Sun, and thus can differ from the Julian day by as much as 8.3 minutes (498 seconds), that being the time it takes the Sun's light to reach Earth. To illustrate the ambiguity that could arise, consider the two separate astronomical measurements of an astronomical object from the earth: Assume that three objects — the Earth, the Sun, and the astronomical object targeted, that is whose distance is to be measured — happen to be in a straight line for both measures. However, for the first measurement, the Earth is between the Sun and the targeted object, and for the second, the Earth is on the opposite side of the Sun from that object. Then, the two measurements would differ by about 1000 light-seconds: For the first measurement, the Earth is roughly 500 light seconds closer to the target than the Sun, and roughly 500 light seconds further from the target astronomical object than the Sun for the second measure. An error of about 1000 light-seconds is over 1% of a light-day, which can be a significant error when measuring temporal phenomena for short period astronomical objects over long time intervals.